1. Field of the Invention
This invention relates to methods of installing elongate underwater structures. In particular, the invention relates to methods of making elongate underwater structures and also to methods of laying seabed pipelines from a surface vessel and to vessels for laying underwater pipelines.
2. Background Prior Art
In the production and laying of rigid conduits, e.g. steel pipe for the transportation of oil or gas, from a vessel on to the seabed there are three basic known methods as follows:
1. S-lay PA1 2. J-Lay PA1 3. Reel-lay
In this method the conduit leaves the lay vessel in a substantially horizontal orientation, bends downwards over a supporting structure, the so-called stinger, and when approaching the sea bottom bends upwards to be laid on the sea bottom. The sideview of the pipeline being laid has an extended S form, the actual shape dependent on pipe weight, water depth and the tension applied to the conduit. A typical example of S-lay is disclosed in U.S. Pat. No. 3,715,890.
The main advantage of the S-lay method is its relatively high production speed particularly in the case where the conduit is composed of steel pipes to be welded together and the vessel is long enough to deploy an economic number of welding stations. The main disadvantage is that for very deep water either the tension to be applied to the conduit has to be very high or a stinger of such a size is required that it cannot easily be supported on a typical lay vessel.
The conduit leaves the lay vessel in a downwardly inclined or even near vertical orientation in order to allow the laying of pipelines in deep water without stressing the pipeline material excessively. This method is explained in U.S. Pat. Nos. 3,266,256 and 3,389,563 to which reference should be made. It should be noted that the method can also be applied in shallow water as illustrated in the article "The one-ship work fleet", published in Ocean Industry of March 1970, pages 52-54.
The main disadvantage of this method is that only a relatively slow production speed is possible because usually only one welding station can be worked on at a time which reduces the production speed significantly compared with the S-lay method where welding is done in a number of welding stations.
Reel-lay can be considered as a variation of J-lay because the conduit leaves the lay vessel in an inclined or even near vertical orientation. The method uses a storage reel on which conduit is stored to be unreeled at site. This method gives a very high lay speed and is obviously advantageous when flexible conduits or cables have to be laid. Examples of this method are disclosed in U.S. Pat. No. 3,389,563 (FIG. 4) and in U.S. Pat. No. 4,340,322 which discloses a self-propelled reel pipe laying ship. A disadvantage of this method relates to the limited capacity of the reels, necessitating regular reloading or change out of reels when a long pipeline is being laid.
Ideas to overcome some disadvantages related to the above mentioned lay methods and combining the advantages of the same are disclosed in International Patent Publication No. WO 95/25237. To limit the bending radius of the steel pipe and hence to limit the diameter of the reel or other curved guiding means, a certain plastic deformation of the pipeline is allowed. This is normal procedure when using the reel-lay method but does require expensive straightening means to straighten the pipeline after the pipeline has left the reel or any other curved part which includes plastic deformation in the pipeline. Other disadvantages of the proposed methods in WO 95/25237 are the necessity to bend and straighten the pipeline more than once (as shown in FIGS. 4 and 5) or that a certain offset angle is required to allow the vertical part of the pipeline to pass the horizontal part of the pipeline.